Silicon Carbide Side Lining

There were no significant results in the application of carbon side-wall materials with enhanced characteristics in the reduction cells, and the current technical decision is silicon carbide side lining. Yet it is necessary to mention that a carbon side lining is in use in low-current cells, especially in Soderberg technology. 

Attempts to use silicon carbide as a side lining material started in the 1980s when the problems with carbon side lining began at the reduction cells with increased current. The technical solution was the application of nitride-bonded silicon carbide. Within the next 20 years, several variants of SiC side lining designs were tested (Fig. 2.64a-f)  

In the course of the trials, designers came to the variant of SiC block that is 60-100 mm thick without heat insulation with a carbon artificial side ledge, preventing the middle and lower parts of the SiC block from contact with the bath and aluminium. The upper part of the carbon artificial side ledge was slightly above the border cryolyte-aluminium. This is the typical construction (2.64e). The continuation of this design with an artificial side ledge was the combiblock (2.64f). 

As we know, carbon side-wall blocks are installed side to side. The thickness of carbon side-wall blocks is 150-200 mm, while the thickness of SiC blocks is sufficiently smaller. There were trials of installing SiC side-wall blocks having a 

Step (Fig. 2.65a) or a rounded and curved side surface (Fig. 2.65b) in order to prevent electrolyte leakage through the shaft. The trials showed that the flat side to flat side variant with mortar between the blocks, covered by a side ledge, provides normal operation of the cell. 

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Fig. 1.1 The records of (a) open porosity of nitride-bonded silicon carbide side lining refractories (in the customer s specification, the open porosity should be below 18 %) (b) the cold crushing strength of nitride-bonded silicon carbide side lining refractories (in the customer s specification, the cold crushing strength should be above 150 Mpa)...

Volume effects of the reactions should also be taken into accotmt (not only in gas corrosion of refractories). The reactions of oxidation of nitride-bonded silicon carbide side lining are positive. The positive volume effect of the reaction may play a positive role, diminishing the open porosity (Table 1.11). However, on the other hand, it may cause tensile strains, which may result in either spalling or cracking of the refractory (Fig. 1.26). 

The immersed tests are usually produced in big crucibles, filled with at least one corrosive agent. The dimension of the tested rods (usually rectangular bars) is 25 mm x 25 mm x 100 mm or 10 mm x 10 mm x 150 mm (Fig. 1.33). The bars may rotate or be dipped and pulled out or may be tested without rotation. The corrosion resistance tests for silicon carbide side lining materials are discussed in detail in Chap. 2. The corrosion resistance tests for A1 melt are discussed in Chap. 3. The tests for sodium swelling of carbon materials are discussed in Chap. 2, devoted to the lining materials for electrolysis, rather than in the chapter devoted to corrosion resistance. 

The electric furnaces, in which this reaction is carried out, are ca. 15 3 3 m- in size and are lined with refractory material. Electrodes at opposite ends are connected to a graphite core. The furnace is filled round this core with the reaction mixture and electrically heated to 2200 to 2400°C. The heating up time is ca. 18 h and the reaction time a further ca. 18 h. After cooling, the sides of the furnaces are removed and the unreacted material on the edges removed. The silicon carbide, which has formed round the graphite core, is broken up and separated into different qualities. 

The side lining of today may contain graphite or preferably silicon carbide to obtain high heat loss in high-amperage cells, but still a frozen ledge is needed for protection. 

The side lining is made from silicon carbide blocks or from carbon side-wall blocks. 

The side-wall blocks from carbon or silicon carbide are placed on a refractory shoulder side line. This side line is above the refractory layers under the cathode bottom blocks. It may be made of bricks or from castables, usually together with heat insulation boards. This sideline has two purposes. First, side lining is installed in the refractory side line, and, second, it helps to compensate for the mechanical tensions due to the sodium swelling (and thermal expansion) in carbon bottom blocks. Heat insulation materials are easily deformed due to tension, but the construction of the refractory lining remains tmdamaged. 

The side lining materials of reduction cells are carbon blocks and silicon carbide blocks. Anthracite-based carbon side blocks were used for many years. As a continuation of R D in the application of carbon side blocks, many approaches were offered (Table 2.12) ... 

Variants of Silicon Carbide Materials (and Others) as a Side Lining... 

Nitride-bonded silicon carbide (silicon nitride-silicOTi carbide material) is not the only variant for application as a side lining material. Here is a list of variants ... 

As shown in laboratory experiments, nitride-bonded SiC is sufficiently more corrosive-resistant, comparing with aluminosilicate-bonded SiC (sintered in air). Currently, nitride-bonded silicon carbide is the main and only material for SiC side lining. 

Silicon Carbide Mortars, Ramming Mixes, and Castables for Installation of SiC Side Lining... 

Figure 3 Phase diagram of silicon carbide (a) big scale (b) silicon side of the phase diagram (c) carbon side of the phase diagram (d) liquid line of the SiC-Si system (e) silicon and carbon interaction parameter ttL in the solution-melt (f) temperature dependences of vapor pressures of main components over 6H-SiC (—SiC-Si,-----------------SiC-C).

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